Buses leave every half an hour or less for Chennai. In this some take the ECR route and lesser numbers take the old route which goes by the Airport.So if you can make a visit to the bus terminus a day earlier and enquire the timings of the old route buses ; will save you a lot of bother on the day of the travel. As per my guess if you catch the 3.30 AM bus you should do fine. Even the ECR route bus can be taken but then you have alight in Thirvanmiyur and take an auto rickshaw ( you can get the information from the bus conductor too as to where you can catch a connecting bus or auto for the Airport). Maybe 3AM would be safer to start your bus travel.